Current polymer production often involves the use of toxic solvents, which place workers at risk, damages the environment, and places a large regulatory burden on government and companies alike. Water-based or solid powder-based production of biopolymers and composites could provide enormous reductions in the toxic solvent load on workers and the surrounding environment. This increased (environmental) awareness has given the materials community impetus to develop cost-effective biomaterials with adequate mechanical properties. While research in recent years has led to an improved understanding of the properties of natural fibers, the problem of identifying a cost-effective biopolymer matrix material or composite material with suitable properties remains unresolved.
Plant proteins, such as for example wheat proteins, are interesting renewable raw materials and already a wide variety of biopolymers based on plant proteins has been used and investigated, alone or in mixtures, in order to obtain for example edible films and coatings. The plant proteins investigated include soy proteins, corn zein, wheat proteins, cotton seed proteins and pea proteins and can be considered as heteropolymers.
Wheat gluten is a mixture of monomeric proteins (gliadins) and “polymerized” proteins (glutenins) linked through intermolecular disulfide bridges. The gluten proteins are largely implicated in the viscoelastic character of gluten and gluten proteins are responsible for giving wheat flour dough its strength and visco-elastic properties. Wheat gluten can easily be isolated by removing starch and water solubles by gently working a dough under a small stream of water. After washing, a rubbery ball is left, which is called gluten. Next to this process which is called “dough or Martin process” other isolation methods exist like the “batter process”. Commercially available gluten contain approximately 75% protein, 10% carbohydrate, <10% moisture, 5% lipids and <1% minerals, but these amounts are variable. The gluten proteins are furthermore very rich in glutamine and proline.
There is already much literature on the use of gluten, also in industrial applications. For example wheat gluten films have been studied in significant detail (Gennadios, A., and Weller, C. L., Food Technol. 1990, 44, 63-69; Gontard, N., et al., J. Food Sci. 1992, 57, 190-195; Herald, T. J., et al., J. Food Sci. 1995, 60, 1147-1150; Roy, S. et al., J. Food Sci. 1999, 64, 57-60; Larré, C., et al., J. Agric. Food Chem. 2000, 48, 5444-5449). Many attempts have been made to convert wheat gluten or corn zein into a usable biodegradable polymer (Guilbert, S. et al., Food Add. Contam. 1997, 14, 741; Pommet, M. et al., Polymer 2003, 44, 115; Redl, A. et al., Rheol. Acta 1999, 38, 311; Pouplin, M. et al., Agric. Food Chem. 1999, 47(2), 538-543; di Giola, L. et al., Agric. Food Chem. 1999, 47, 1254). Films have been cast from gluten protein dispersions in water under different pH conditions or in ethanol. It was demonstrated that plasticizing agents could be used to improve film flexibility and decrease brittleness (Ali, Y et al. Ind. Crops Prod. 1997, 6, 177-184). Indeed, researchers have observed that the preparation of wheat gluten films necessitates the use of a plasticizer. In the absence of a plasticizer, gluten films are brittle and difficult to handle. A number of plasticizers have been explored in the past, including amines (diethanolamine and triethanolamine) and polyhydroxy compounds (anhydrous glycerol, polyethyleneglycols, and polypropyleneglycols). Typical concentrations range from 10 g to 60 g/100 g of dry matter (Gennadios, A. and Weller, C. L., Food Technol. 1990, 44, 63-69; Roy, S. et al., J. Food Sci. 1999, 64, 57-60). From three known plasticizers (water, glycerol and sorbitol), water was found to be the most effective plasticizer (Pouplin, M., et al., J. Agric. Food Chem. 1999, 47, 538). The action of a plasticizer is generally to interpose itself between the polymer chains and alter the force holding the chains together. Polymer plasticization enables thus to reduce the shaping temperature of the thermoplastic process and to impart adequate flexibility to the material. However, it can also greatly influence the functional properties of the material (Pommet, M. et al., Polymer 2003, 44, 115). In a more recent study, Pommet et al. explored the use of fatty acids as a plasticizing agent for gluten (Pommet, M., Redl, A., Morel, M. H., Guilbert, S., Polymer 2003, 44, 115). Their thermo-mechanical data revealed a “compatibility limit” between the lipid and gluten, beyond which phase separation was observed.
The prior art describes methods for fractionating gluten into gliadin and glutenin (Midwest Grain U.S. Pat. No. 5,610,277) and attempts to form solid, non-edible biodegradable, grain-protein based articles (Midwest Grain U.S. Pat. No. 5,665,152), where the processes applied always involves cleaving of the disulfide linkages in the protein using at least 0.01 by weigth of a reducing agent, selected from sodium sulfite, sodium bisulfite, sodium metabisulfite or ascorbic acid and respectively furthermore selected from alkali metal and ammonium sulfites, nitrites, mercaptoethanol, cysteine, cysteamine and mixtures thereof. The formulation of U.S. Pat. No. 5,610,277 also includes from about 20-85% by weigth of grain protein, from about 5-75% by weigth starch, up to about 14% by weigth water and from about 10-40% by weight of a plasticizer, such as glycerol. In the U.S. patent, also the mixing with fibers is described. While some aspects of the work by Midwest Grain appear similar to our work, there are however several distinct differences. That is, the additive we propose in first instance not only reduces a number of the disulfide linkages in the protein, but they also have the potential to be covalently incorporated in the protein polymer network and crosslinking the proteins, thereby modifying the polymer network and as a consequence modifying the material properties.
It can be argued that the formation of covalent bonds is a necessary first step in the development of a stable biopolymer system. Most of the studies presented thus far have relied on plasticizers that, at best, form only hydrogen bonds with the gluten protein chains (see references above). The use of chemical crosslinkers to modify the properties of protein-based materials have been reported as well, but to induce crosslinking with the protein structure required either the use of a catalyst (Ghorpade, V. M. et al., Trans ASAE 1995, 38, 1805; Larré, C., Agric. Food. Chem. 2000, 48, 5444) or an aggressive radiation treatment (Brault. D., Agric. Food. Chem. 1997, 45, 2964).
Thus, much research has already been undertaken in order to obtain a gluten or zein based usable biodegradable polymer. However, all of these approaches encounter problems and a usable biodegradable polymer has not been described yet. For example, previously reported experiments, designed to improve the impact strength of the gluten material require the addition of at least 10-20% (w/w) of some plasticizer, such as glycerol or triethanolamine.
As a summary, there is still a great need for cost-effective biomaterials with adequate mechanical properties. Therefore, a goal of the present invention is to satisfy this need by developing a new biopolymer and biodegradable composite material with interesting properties such as an increased strength and toughness, by identifying a method for increasing the impact strength of natural proteins and fibers and by producing new biodegradable composite materials. This invention describes a method to improve the impact properties of gluten biopolymer, enabling broader usage of gluten in industrial applications. This invention also describes a new composite material and a process to make fully biodegradable composite materials.
Polythiols are molecules with multiple thiol groups in their structure and they are used for many reasons such as in the production of different polymers. The applications of these polymers include compositions for special coatings, inks and optical devices. Polythiols as crosslinking agents often influence the thermal and mechanical properties of the resulting polymers. In the prior art however, polythiols have not been used with natural polymers such as gluten.